It’s giant hogweed season and experts are once again warning Canadians to exercise caution to avoid being burned by the hazardous plant.
The invasive species can cause severe injuries, including temporary vision loss and painful blisters, and it is a threat to local ecosystems.
Here’s what you need to know about the plant with a toxic sap that can burn and blind people.
What is giant hogweed and what does it look like?
Heracleum mantegazzianum, commonly known as giant hogweed, is an invasive species that is considered one of Canada’s most dangerous plants, according to the Nature Conservancy of Canada.
The plant’s sap contains toxic substances that react with sunlight and artificial UV light, causing skin lesions akin to severe burns.
Characterized by its towering height of up to six metres, giant hogweed features large clusters of white flowers atop an umbrella-shaped head. Its hollow, rigid green stem may exhibit dark red or purple spots, while its dark green leaves have many points.
Giant hogweed thrives in cool and moist environments and is commonly found along river banks, ditches, railway tracks, roadsides, and in meadows and vacant lots.
According to the Ontario Invasive Plant Council, giant hogweed has a scattered distribution across southern and central Ontario, south of the line from Manitoulin Island to Ottawa. Extensive populations have been found within the Greater Toronto Area, with confirmed sightings as far north as Kapuskasing.
Limited populations of giant hogweed also exist in Quebec, New Brunswick, Nova Scotia, Newfoundland and Labrador, and British Columbia.
Why is giant hogweed dangerous?
The sap of giant hogweed contains a chemical that triggers a condition known as phytophotodermatitis. This condition renders the skin extremely sensitive to sunlight, leading to severe burns and blisters upon exposure to UV rays.
“It produces large quantities of sap containing a chemical that reacts with sunlight and can cause third-degree burns and blistering when it comes into contact with people’s skin,” said Andrew Holland, national spokesperson with the Nature Conservancy of Canada.
A 14-year-old was hospitalized for three days in Ireland after giant hogweed sap got on his skin while he was at a neighbour’s house,the Daily Mail reported in July.
Giant hogweed was introduced to Canada from Eastern Europe and Asia in the 1940s as a decorative plant, said Holland.
“A single giant hogweed plant can produce thousands of seeds per year and it can spread quickly. The seeds are dispersed when they fall into rivers and streams and can be dispersed short distances by the wind,” he said.
Its height and perennial nature allow it to dominate areas along rivers and streams, depriving native vegetation of space, sunlight and water.
Human activities, such as unintentional transport of seeds or international cultivation, contribute to the plant’s spread.
“One issue is that people unknowingly or unintentionally think it would be an interesting garden addition and move it from garden to garden or collect the seeds and plant them,” Holland said.
How to distinguish giant hogweed from cow parsnip and wild parsnip
Cow parsnip and wild parsnip, both members of the carrot family, are often mistaken for giant hogweed due to their similar appearance. However, the former two are not an invasive species.
Cow parsnip generally does not grow taller than three metres in height, half the height of giant hogweed. Its stem is covered with soft, felt-like white hairs, occasionally displaying a red tinge. The leaves of cow parsnip have a felt-like texture and are abundantly covered with soft, woolly white hairs.
Like giant hogweed, cow parsnip can cause itching, burning and blistering if it comes into contact with the skin.
Wild parsnip is more common than giant hogweed. It can be easily differentiated by its clusters of bright yellow flowers instead of white flowers, said Holland.
Wild parsnip is commonly found along roadsides and can cause a reaction similar to poison ivy.
What to do if you are exposed to giant hogweed sap
If exposed to giant hogweed sap, it is crucial to take immediate precautions to minimize the adverse effects. The Nature Conservancy of Canada recommends the following:
Avoid sunlight and artificial light, as it will make the reaction worse.
Use a paper towel to remove sap without rubbing the affected area, taking care not to spread it further.
Thoroughly clean the skin with soap and water to reduce the severity of the reaction. Symptoms can manifest up to 48 hours after exposure.
Wear protective gloves, long pants and a long-sleeved shirt for at least 48 hours.
In case of sap contact with the eyes, rinse them thoroughly with clear water for at least 10 minutes and seek medical attention. Wear sunglasses with dark lenses to provide protection against light exposure.
If burns occur, cover the affected area for a week and use sunscreen with an SPF of at least 30 for six months.
Consult a doctor if a child has come into contact with the sap, if sap enters the eyes, if a rash or blisters occur or if the affected person develops a fever.
What to do if you see giant hogweed
The Nature Conservancy of Canada said that people should contact their local municipality or provincial invasive plant and species councils to report sightings.
Canadians are asked to upload photos of the plant on the iNaturalist.ca app, which monitors its spread.
